In recent years, automobiles have sophisticated multiple functions. Consequently, a large number of sensors have come into use in order to obtain various vehicular information items. The sensors are respectively connected to a vehicular control apparatus through wire harnesses. Therefore, the number of the wire harnesses increases and cause problems of heavier weight and higher cost. On the other hand, it is proposed that a plurality of sensors is connected to a vehicular control apparatus through common communication lines. Here, various detected vehicular information items are transmitted to the vehicular control apparatus by serial communication. Such a communication apparatus is disclosed in, for example, JP-A-2003-32159.
This serial communication apparatus includes a master controller, one set of communication lines, and a plurality of slave controllers. The master controller serially transmits slave-controller specifying information items and request information items to the plurality of slave controllers through the communication lines. The slave controllers corresponding to the slave-controller specifying information items serially transmit the information items corresponding to the request information items to the master controller through the communication lines.
Meanwhile, when the number of sensors increases still more, a plurality of sets of communication lines L1, L2 to which a master controller M10 and a plurality of slave controllers S11 to S14 and S21 to S24 are connected as shown in FIG. 8 are employed in order to enhance the transmission efficiency of vehicular information items. In FIG. 8, only two communication lines L1 and L2 are shown. In this case, as shown in FIG. 9 by way of example, the master controller M10 delivers both slave-controller specifying information items SS and request information items SR to the respective sets of communication lines L1 and L2 at the same timing. As shown in FIG. 10, the slave-controller specifying information SS and the request information SR are successively delivered and serially transmitted bit by bit in synchronism with a clock CLK of predetermined period T′ which is generated by the master controller M10. As shown in FIG. 11 by way of example, the slave controller corresponding to the slave-controller specifying information item SS delivers response information item RES corresponding to the request information item SR, to the master controller M10 at a timing at which the next slave-controller specifying information item SS and request information item SR are transmitted. As shown in FIG. 12, the information item corresponding to the request information SR is successively transmitted and serially transmitted bit by bit in synchronism with the next slave-controller specifying information and request information.
As shown in FIG. 13, therefore, the slave-controller specifying information items SS, request information items SR and response information items RES corresponding to the request information items SR are successively delivered and serially transmitted bit by bit at the same timing in all the sets of communication lines L1 and L2. These information items are transmitted as the changes of voltages or currents. Accordingly, the timings of the changes of the respective bits of the information items to-be-transmitted are synchronized in all the sets of communication lines L1 and L2, and the changes of the voltages or currents become large, so that noise develops. The noise will cause adversary influence on radio broadcasting.